Draw-forming of a surface-treated steel sheet such as a tinplate or tin-free steel sheet or a metal sheet such as an aluminum sheet is widely adopted for the manufacture of a so-called seamless can (two-piece can) and a metal cap. Coating or printing of individual formed bodies after the forming operation requires a complicated operation, and therefore, coating or printing of a flat blank before the draw-forming is desirable.
In draw-forming, a flat metal blank undergoes the plastic flow and is formed into a cup-shaped body. When the plastic flow of the portion of the flat blank, that will constitute the circumferential side wall portion of the formed body, is taken into consideration, it is confirmed that in this portion, elongation is caused in the direction corresponding to the height direction of the formed body.
In the preliminary printing of a flat blank for draw-forming, in view of the above-mentioned plastic flow of the blank, it is necessary that an original which is rectangular in the developed state should be converted to an annular printing plate. As means for effecting this conversion, there are known a hand-writing method, an optical conversion method and a digital conversion method proposed by us (Japanese Unexamined Patent Publication No. 61-267763 and Japanese Unexamined Patent Publication No. 61-267053).
However, in a can obtained by draw-forming a preliminarily printed metal blank, a striped pattern not found in the original print image is often formed at the upper part of the side wall portion, and this is especially conspicuous in a deep-draw-formed can or a can in which the thickness of the side wall portion is reduced by bending-rebending deformation.
As the result of the research made by us, it was found that in the draw forming of a preliminarily printed metal print, at the upper part of the side wall portion, dots become continuous owing to the compression flow of the blank in the circumferential direction and spaces between dots in the height direction increase owing to the elongation flow of the blank in the height direction, and this deformation of the dots tends to result in formation of a striped pattern.
Furthermore, the above-mentioned prior art techniques are defective in that the object is limited to an ordinary draw-formed can.
In the draw-redraw forming of a covered metal sheet, there is known a method in which the metal sheet is draw-formed into a deep-draw-formed cup by a curvature corner portion of a redrawing die and the side wall portion is subjected to the bending-rebending deformation to reduce the thickness. However, in the production of this thickness-reduced draw-formed can, there is not known a method in which the metal blank used is preliminarily printed and a print image faithful to the original image is manifested.
According to the above-mentioned hand-writing method, a rectangular transparent original ruled in squares is wound around the circumferential side wall portion of the draw-formed body, areas corresponding to the original image are painted by hand writing on the annular developed plane having squares correspondig to those of the ruled original to prepare a printing plate for a sheet-shaped blank. However, a high skill is necessary for the preparation of this printing plate and a large cost and a long time are required. Moreover, the print image practically reproduced on the circumferential side wall portion of the draw-formed body is still insufficient in the precision and the like.
The technique of forming a printing plate by optically developing a rectangular original annularly has already been proposed. For example, Japanese Examined Patent Publication No. 45-11388 discloses a method in which a rectangular original is photographed so that the height direction is shortened, the obtained photo is wound around a mandrel and the photo is photographed again through a frustoconical mirror arranged around the mandrel to obtain a printing plate having an annularly developed image, and Japanese Examined Patent Publication No. 48-23455 discloses a method in which a frustoconical prism having at the center a hole having an inner diameter almost equal to the outer diameter of the can is arranged, a rectangular original is located on the inner surface of the hole and this original image is photographed by a camera through the above-mentioned prism and a correcting lens to form a plate having an annular plane.
In these methods, however, troublesome operations and particular optical devices are necessary, and because of the abberation or the like of the optical system, it is difficult to reproduce a fine dot pattern at a high precision. Moreover, every time the original for the draw-formed body is changed, the troublesome conversion operation should be carried out for forming a plate.
In each of metal blanks for cans, the thickness is reduced by a high rolling operation, and an anisotropy is produced in the plastic flow of the blank at the draw-forming by an influence of this rolling. Accordingly, turbulence of a print image is readily caused in the side wall portion of the obtained draw-formed can, especially at the upper part of the side wall portion, and the precision or linearity of the image is often degraded.